Laser diode module

ABSTRACT

In a wavelength locking optical system, a resonator is formed between a wavelength deviation detection device and a facet of the laser diode, which makes a control signal unstable. A solution to this problem is to arrange polarization controlling elements between a wavelength deviation detection filter and a laser diode to stabilize the deviation signal, and to perform wavelength-locking with the stabilizer signal.

BACKGROUND OF THE INVENTION

The present invention relates to an optical communication module that isapplied to a wavelength division multiplexing optical communicationsystem. That is, the invention provides a stable optical system forlocking a lasing wavelength of light from a laser source and a controlsystem for the optical system. This optical system can be separatelyoperated as a wavelength locker module, but it also can be integratedinto an optical communication module having a laser source.

Optical fiber communication features a long transmission length, highspeed and large capacity, and a strong immunity to electromagneticnoises; and, hence, a communication system that assures high reliabilitycan be provided. Formerly, in such systems, light of a single wavelengthwas transmitted on a single strand of optical fiber. However, with theadvent of large capacity computerization in recent years, there has beena strong demand for the transmission capacity to be further increased.Therefore, a wavelength division multiplexing optical communicationsystem has been developed and put into practical use, in which aplurality of optical signals each having a different wavelength aretransmitted over a single strand of optical fiber, so that the number ofcommunication channels is increased to achieve a system having a largercapacity. Normally, for the wavelength of light to be transmitted in anoptical fiber, use is made of wavelength bands where the transmissionloss of the optical fiber is low, and such wavelength bands in a 1.3 μmrange and in a 1.5 μm range are called windows of transmission. Sincethe wavelength widths of these windows are limited, the narrower thewavelength spacing between adjacent channels becomes, the more thenumber of transmission channels can be increased. Presently, thefrequency spacing is set to 200 GHz and 100 GHz, but there is a trendtoward further narrowing of the frequency spacing, such as to 50 GHz and25 GHz. Converting the above-mentioned frequency spacings intowavelength spacings, those values become as narrow as approximately 1.6nm, 0.8 nm, 0.4 nm, and 0.2 nm. When the wavelength spacing is narrowedto such levels, it becomes necessary for the wavelength of the lasersource to be controlled to a constant value with pinpoint accuracy. Thisis because, if the wavelength of the laser source fluctuates to reach asfar as the wavelength of the adjacent channel, there occurs crosstalkwith the adjacent wavelength channel at the reception side, and, hence,the reliability of information transmission can not be assured. Thesewavelength (or, frequency) channels are called ITU-T (INTERNATIONALTELECOMMUNICATION UNION-TELECOMMUNICATION STANDARDIZATION SECTOR) gridsand are acknowledged widely as an ITU recommendation.

On the basis of the aforementioned considerations, there have beenproposed several methods for controlling the wavelengths of the lasersources for the wavelength division multiplexing of opticalcommunication systems. For example, a method has been devised forlocking the wavelength of the laser diode by introducing a dielectricmulti-layer filter, a Fabry-Perot etalon, or the like as a wavelengthfilter and using feedback to control the wavelength on the basis of theoperating temperature of the laser diode. Among these wavelengthfilters, especially the etalon has characteristics such thattransmission peaks appear repeatedly in the wavelength according to thenumber of orders of multi-interference, and therefore, by adjusting theperiods of the transmission curve to the ITU-T grids, a singlewavelength filter can be used to lock a plurality of wavelengthchannels. For example, JP-A-79723/1998 discloses a method of locking thewavelength by dividing light which has passed through the etalon intotwo portions, detecting the two portions using respective photodetectors, and subtracting one signal from the other signal to derive awavelength deviation signal, which will be used to lock the wavelength.

SUMMARY OF THE INVENTION

It is a first object of the present invention to stabilize thewavelength of a laser diode. More specifically, it is an object of theinvention to achieve stabilization of a wavelength locking systemutilized in a semiconductor laser module (hereinafter referred to as a“laser diode module”) in which the wavelength locking optical system isincorporated.

Among the transmitted light beams of the etalon utilized by thewavelength locking system, any light beams that contribute to thewavelength deviation detection effectively are almost collimated lightbeams. Accordingly, light reflected from the etalon goes back to thelaser diode via a converging lens, is reflected by a facet thereof, andis reflected again by the etalon, which is repeated to cause multiplereflections. Therefore, the reflected waves multiply-interfere with oneanother; and, consequently, a distribution of the interference fringesvaries in response to variation of the wavelength. Thus, light thatarrives at a photo detector fluctuates, thereby to generate a ripple inthe output, and so there arises a problem of instability in thiswavelength deviation signal.

It is a second object of the present invention to eliminate or alleviatethe external feedback noise in a laser diode. That is, in the operationof a laser diode, there is the problem that so-called external feedbacknoise (returned light noise) is generated, namely a fluctuation in thelasing intensity resulting from contention of the lasing mode of thelaser diode itself with an external resonance mode that is generated bylight returned to the laser diode being coupled to a waveguide thereof.

A basic form of the present invention consists of a laser diode modulethat comprises at least a laser diode device; a first detector elementfor receiving, directly or indirectly, a first light beam that isobtained when at least one of the light emissions of said laser diodedevice is divided into two light portions, each traveling in a differentdirection; a second detector element for receiving a second light beamthat comprises the other of the divided light beams at least via awavelength selective member; and means for controlling the lasingwavelength of the above-mentioned laser diode device on the basis ofoutputs of the above-mentioned first and second-photo detector elements,wherein a gap between the above-mentioned laser diode device and theabove-mentioned wavelength selective member is formed so as toconstitute an optical resonator with multi-interference eliminated oralleviated therein.

The following description provides typical examples of the constructionof an optical cavity in which multi-interference between theabove-mentioned laser diode device and the above-mentioned wavelengthselective member is eliminated. The first scheme involves a technique inwhich the polarization directions of the light emitted from the lightsource and of light returning to the light source, as returned light,are made different from each other. A more concrete example is asfollows. The above-mentioned wavelength selective member in an opticalpath between the laser diode and the above-mentioned wavelengthselective member itself is arranged so as to generate reflected lighthaving a degree of polarization different from that of the incidentlight falling on the wavelength selective member. In the most preferableform, this difference between the directions of polarization is suchthat the directions of polarization are mutually orthogonal, which canensure the most stable operation.

The second scheme involves reduction of the mutual superposition of alight beam emitted from the light source and a light beam that returnsto the light source as returned light. For example, the angle of thereflection surface is tilted to avoid a superposition of the incidentlight and the reflected light. A straightforward configuration for thisscheme is as follows. When the incident light is vertical to thesurface, the surface is tilted with the respect to the optical axis ofthe incident light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wavelength control loop;

FIG. 2 is a graph showing an example of a relation between the lasingwavelength of a tunable laser diode and the temperature;

FIG. 3 is a diagram showing a principle of operation of an etalonaccording to the present invention;

FIG. 4 is a graph showing a wavelength spectral characteristic of theetalon according to the present invention;

FIG. 5 is a diagram showing an example of a wavelength locking opticalsystem according to the present invention;

FIG. 6 is a diagram showing an example of the wavelength locking opticalsystem according to the present invention;

FIG. 7 is a diagram showing an example of the wavelength locking opticalsystem according to the present invention;

FIG. 8 is a diagram showing an example of the wavelength locking opticalsystem according to the present invention;

FIG. 9 is a diagram showing an example of the wavelength locking opticalsystem according to the present invention;

FIG. 10 is a diagram showing an example of the wavelength lockingoptical system according to the present invention;

FIG. 11 is a sectional view showing an example of a wavelength lockedoptical module according to the present invention;

FIG. 12 is a diagram showing an example of a wavelength locked opticalmodule, wherein tunable laser diodes, whose wavelengths are locked bythe wavelength locking optical system according to the presentinvention, are arranged side by side to achieve enlargement of a tunablewavelength range;

FIG. 13 is a diagram showing a sub-assembly to be used in the opticalcommunication module; and

FIG. 14 is a diagram showing an example of the optical communicationmodule into which a wavelength locking unit according to the presentinvention is built.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As can be understood from the foregoing description, in order to reducemulti-interference in the resonator that is formed between the facet ofthe laser diode and a wavelength deviation detection filter, arepresentative embodiment of the present invention adopts aconfiguration wherein the facet of the laser diode and a wavelengthdeviation detection filter are arranged so as not to be parallel witheach other, or wherein, by arranging a wave plate inside the resonator,the polarization states of the light beams are controlled so as toreduce superposition of the light beams having the same polarization;and, the arrangement further includes means for preventing multiplereflection by use of a polarizing element. Hereafter, several forms bywhich the objects of the present invention can be carried out inpractice will be described as examples.

FIG. 1 shows an example of a control loop that acts to carry outwavelength locking according to the present invention. That is, thelaser source 100 is a tunable laser diode device, for example, a DFB(distributed feed back) type or Fabry-Perot type laser diode device.Alternatively, the laser source 100 is a light source in which a DFBtype laser diode device and an electro-absorbing modulator areintegrated. Alternatively, the laser source 100 is a light source inwhich a laser diode device, equipped with a temperature controlmechanism for tuning the wavelength, and an electro-absorbing modulatorare integrated. That is, the tunable laser source 100, a collimator lens105, a beam splitter 106, a photo detector 110, an etalon 108, a photodetector 109, a thermistor 403, etc. are mounted on a thermoelectriccooling device 401 placed below a stem 300, and the temperature of thelaser 100 can be kept at a temperature in accordance with the resistancevalue of the thermistor 403 by a driving circuit 402 of thethermoelectric cooling device 401. on the other hand, forward emittinglight of the tunable laser source 100 is collimated by a collimator 101and coupled to an optical fiber 104 via an optical isolator 102 and aconverging lens 103 to travel therein for information transmission.

This wavelength controlling loop has a structure in which, by switchinga temperature control loop 488 that has been closed by a switch 405 to awavelength control loop 489, the wavelength control loop is selected.First, backward emitting light from the tunable laser source 100 iscollimated by the collimator 105 and divided into transmitted light 107and reflection light by the beam splitter 106 (a device for dividinglight into two portions and guiding the two portions to differentoptical paths). The beam splitter 106 is of a shape formed by gluing twotriangular prisms, or comprises a thin parallel planar plate with adielectric thin film deposited thereon, if a non-glued structure isfavored. The reflected light undergoes photoelectric conversion tobecome an output monitor signal of the laser source 100, which iscompared with a previously-set value of the optical output and is fedback to a driving circuit 404 to keep the output value of the lasersource constant. Further, the optical system operates to detectwavelength deviation according to the present, invention as follows.Since, in the tunable laser source 100, the lasing wavelength t variesas the temperature T changes, as shown in FIG. 2, the lasing wavelengthcorresponding to the ITU-T grid can be selected in accordance with theoperating temperature. In FIG. 2, the horizontal axis denotes thetemperature of the laser diode and the vertical axis denotes the lasingwavelength. It is a general application that the lasing wavelengths ofthe laser diode correspond to the wavelengths which are defined by theITU-T grids. This matter will be described later. Incidentally, in theexample of FIG. 2, the variation of the wavelength as a function of thetemperature is 0.1 nm/° C. In this control loop of FIG. 1, the forwardemitting light of the tunable laser source 100 is collimated by thecollimator 101, passes though the optical isolator 102 and a converginglens 103, and is coupled to the optical fiber 104. Thus, information istransmitted on the light.

The configuration of an optical system for wavelength locking is asfollows. That is, backward emitting light of the tunable laser source100 is collimated by the collimator 105 and is divided into transmittedlight 107 and reflected light by the beam splitter 106. For the beamsplitter 106, there is a geometry in which triangular prisms are gluedtogether or a thin parallel planar plate with a dielectric thin filmdeposited thereon is used. If a non-glued structure is favored, thelatter is useful. The transmitted light 107 further travels to theetalon 108, and inside the etalon 108, light waves generated by multiplereflections interfere with one another to create transmission peaksexpressed by the following equation (1).

It=1/(1+F sin²(ψ)),  (1)

where

F=4R/(1−R)2  (2)

ψ=(2θnt/λ)cos θ′  (3)

As shown schematically in FIG. 3, the light incident on the etalonhaving a refractive index n at the point A1 is divided here intoreflected light R1 and light which enters the etalon. The entered lightis further divided into light reflected at the point B1 and light T1transmitted to the outside of the etalon. Reflected light generated atthe point BE includes light which will be emitted to the outside of theetalon through the opposite surface thereof as light R2 and light thatis further reflected and emitted to the outside thereof as light T2.Thus, reflections are repeated inside the etalon to produce multiplereflections.

FIG. 3 illustrates how to measure several quantities in the equationsdescribed above. As shown in FIG. 3, R denotes the surface reflectivityof either surface of the etalon 108, n denotes the refractive index ofthe etalon, t denotes the thickness of the etalon, λ denotes thewavelength of light, and θ denotes an angle of light rays to the normalinside the etalon.

Here, Snell's law applies:

sin θ=n sin θ′  (4)

In addition, the spacing of the multiple peaks in the transmitted lightis called the free spectral range (FSR) which is expressed in thewavelength range by

FSR−λ ²/2nt  (5)

and in the frequency range by

FSR−c/2nt  (6),

where c denotes the speed of light. Therefore, if the etalon isoptimally designed with the thickness t, the refractive index n, and thesurface reflectivity R, the FSR can be made to agree with the ITU-T gridspacing. Setting the FSR to the ITU-T grid makes it possible to achievewavelength deviation detection with steep wavelength selectivity over awide range of wavelengths. Thus, it is possible for a plurality oftransmission ranges of light which exist with a predetermined wavelengthspacing to exist, and in which one of the transmission ranges is matchedto a desired wavelength of the above-mentioned laser source.

FIG. 4 is a graph showing an example of the wavelength spectralcharacteristic of the etalon. In the figure, the horizontal axisrepresents the wavelength shift and the vertical axis represents theoptical output. The transmission peaks in the etalon appear repetitivelywith increasing/decreasing wavelength, as shown in FIG. 4. Further, thespacing of the transmission peaks is called, as mentioned above, thefree spectral range (FSR) and is expressed by the equations (5), (6).Thus, transmitted light of the etalon so obtained is received by thephoto detector 109 in FIG. 1 and is converted into a photoelectriccurrent. On the other hand, the reflected light from the beam splitter106 is received by the photo detector 110 and is also converted into aphotoelectric current.

Then, the difference between photocurrent Pm generated by the photodetector 110 for optical output monitoring, as described above, and thephoto current Pt generated in the photo detector 109 for wavelengthmonitoring, which is in obedience to the equation (1), as a result ofthe effect of the etalon 108, is-assumed as the deviation signal A(λ).

A(λ)=Pt−Pm  (7)

It is also appropriate that, for the deviation signal, a signalnormalized by the light quantity may be used.

A(λ)=(Pt−Pm)/(Pt+Pm)  (8)

Thus, the wavelength deviation signal value A(λ), which was obtainedwith a high sensitivity, is fed back to the driving circuit 402 tocontrol the temperature control element 401 of the laser diode 100 so asto become zero or a constant value. It should be noted here that, inorder for the deviation signal represented by either equation (7) or (8)to cause the wavelength to capture one of the ITU-T grid wavelengths orits vicinity. The temperature of the laser diode is sensed from thevalue of the thermistor 403 beforehand, and the operating temperature isset to a temperature to said wavelength or its vicinity in accordancewith the relation between the wavelength and the temperature obtained asshown in FIG. 2. After this preparation, the switch 405 in FIG. 1 isswitched to the wavelength-locking loop. On the other hand, APC(AUTOMATIC POWER CONTROL) for keeping the power output of the laserdiode constant compares, in a similar way, the output of the photodetector 110 with a set value and the difference thereof is fed back tothe driving current control circuit 404 of the laser diode 100. Theforegoing description concerns the control loop for wavelength locking.

A ratio of the full width of the half maximum t of the transmission peakof the etalon to FSR is called finesse Φ, which has the followingrelations of equations (9)-(11).

F=4R·(1−R)z  (9)

Φ=(n{square root over (F)})/2  (10)

Φ=FSR/ε  (11)

Therefore, in order to achieve a steep spectral characteristic ε of theetalon, it is necessary to increase the reflectivities of both sides ofthe etalon and also to cause reflection at both sides to occur a numberof times. Consequently, the angle of the incident light to the etalonneeds to be set to the vertical angle (incident angle of zero degree) orits vicinity. If the incident angle deviates from the normal incidence,effective finesse decreases because the reflected light at eachreflection constituting part of the multiple reflections is shifted in adirection perpendicular to the-optical axis and the wavefronts of therespective reflections superpose only partially, hence reducing theinterference of light.

However, if the incident angle of light to the etalon becomes close tothe normal incidence light resonates between a rear facet of the laserdiode 100 and the etalon. This resonator can be analyzed by applying amethod similar to that of the etalon that is a key component of theabove-mentioned wavelength locker. Here, setting the reflectivity of thefacet of the laser diode at 0.9, the transmissivity of the collimatorlens at 0.95, the transmissivity of the beam splitter at 0.50, and alsodenoting the reflectivity of the etalon as Ir(λ), the effectivereflectivity that governs the finesse of a newly formed Fabry-Perotresonator is given by

Re-SQRT(0.9×0.95×0.5×Ir(A))  (12)

Ir(λ)=1−It  (13)

The finesse of the external Fabry-Perot resonator is given by putting Rein the equation (9) and (10). For example, if it is assumed thatIr(λ)=0.5, then Re=0.546, which indicates the existence of a resonatorwith a finesse of approximately 4. Therefore, as the wavelength variesringing occurs in the output of the photo detector 110. Since thisphenomenon induces a malfunction both in the APC (AUTOMATIC POWERCONTROL) of the laser diode and in the control loop forwavelength-locking, a countermeasure needs to be devised. This is aproblem solved by the present invention.

A first method of the countermeasure includes reduction in thereflectivity of the facet of the laser diode and blocking the returnedlight onto the facet of the laser diode. Or, the direction ofpolarization of the returned light onto the facet of the laser diode ismade orthogonal to that of the initial light to effect suppression ofmutual interference. That is, a quarter-wave plate 111 is arrangedbetween the collimator lens 105 and the etalon 108, as shown in FIG. 5.The physical relationship of the quarter-wave plate 111 and the beamsplitter 106 is such as to be advantageous optically or inimplementation.

FIG. 5 is a view showing an example of the wavelength locking opticalsystem according to the present invention. This example uses thequarter-wave plate. A detailed explanation of the example shown in FIG.5 is omitted because the basic configuration, other than the use of thequarter-wave plate, is the same as the example of FIG.

For a material of the quarter-wave plate 111, normally, quartz, calcite,etc., all of which have optical anisotropy, can be used. Since normallythe light of the laser diode is almost linearly polarized in thedirection of a waveguide stripe thereof (in a direction parallel to theface of the drawing), the light is given a circular polarization afterpassing through the wave plate 111; and, after being reflected by theetalon 108, it passes through the wave plate 111 again backwards, and itbecomes linearly-polarized light whose direction of polarization isvertical to the face of the drawing. Since these mutually orthogonallinearly polarized light beams do not interfere with each other,occurrence of ringing, which becomes a problem, is reduced. However,light that has returned to the laser diode again and is reflected thereagain becomes linearly-polarized light whose direction of polarizationis the same as that of the initial light produced by the transmissionthrough the wave plate 111 and the reflection at the etalon 108. In thisparticular case, if the loss of the resonator is sufficiently large,mutual interference is minute and the ringing can be ignored, but theexistence of such an expedient case depends on the degree of the loss ofthe resonator in comparison with an allowed value.

FIG. 6 shows an embodiment where the above-mentioned wave plate 111 alsoserves as the beam splitter 106 functionally. Actually, a member 112 inFIG. 6 constitutes the member that serves basically as the quarter-waveplate and doubles also as the beam splitter. Regarding the example ofFIG. 6, a detailed explanation is omitted because the basicconfiguration, other than that of the member 112, is the same as theexample of FIG. 1 or FIG. 5. That is, the normal beam splitter 106 ismade of a vitreous material and hence is optically isotropic. However,as in this example, the beam splitter can be provided with the functionof a quarter wave plate for light of oblique incidence if the beamsplitter is made of an anisotropic material and the thickness thereof isdesigned to be equivalent to that of the quarter-wave plate. Then, bydesigning a coating film on the surface, a beam splitter with a desiredsplitting ratio can be obtained. If such a component is provided, thenumber of the components can be reduced and an effect that improves thedegree of integration in a package can be brought out.

FIG. 7 illustrates a further embodiment according to the presentinvention. That is, the embodiment has a base configuration as shown inFIG. 5 and FIG. 6 with the addition of a polarizer 113. The polarizer113 is arranged between the collimator lens 105 and the quarter-waveplate 111, or the quarter-wave plate 112 also serving as a beamsplitter, with its direction set such that linearly-polarized light ofthe laser diode can be transmitted. The linearly-polarized light havingpassed through the polarizer 113 passes through the quarter-wave plate,is reflected by the etalon, and again passes through the quarter-waveplate backwards to become linearly-polarized light whose direction ofpolarization is orthogonal to that of the initial light (in a directionvertical to the face of the drawing). Since the linearly polarized lighthaving a direction of polarization vertical to the face of the drawingis blocked by the polarizer 113, it does not go back to the facet of thelaser diode. Thus, the above-mentioned resonance is suppressed and theringing in the output of the photo detector associated with thevariation in the wavelength is reduced.

FIG. 8 illustrates other means for similarly separating the returnedlight and the initial light through the use of polarization. That is, itis an embodiment wherein the beam splitter doubles as a polarizer. Thelinearly polarized light from the laser diode is made to enter a beamsplitter 114, as P (parallel) light, which transmits part of the lightand reflects part of the light. This reflected light is received by thephoto detector 110 and serves as part of the output monitor/wavelengthdeviation detection signal. The transmitted light passes through thequarter-wave plate 111 to become circularly-polarized light, isreflected by the etalon 108, again passes through the quarter-wave plate111 so as to have an s (i.e., senkrecht) polarization, and enters thepolarizing beam splitter 114. In this example, the light is reflectedthere by a reflectivity of almost 100 percent, and thus the lightreturned to the laser diode is blocked. Needless to say, the transmittedlight from the etalon 108 arrives at the photo detector 109 to producethe wavelength deviation signal. Such an optical arrangement makes itpossible to suppress the resonance occurring between the facet of thelaser diode and the etalon, and also to close a stable wavelengthcontrol loop because malfunction in the wavelength deviation signal aswell as in the optical output monitor are eliminated.

FIG. 9 illustrates a further embodiment according to the presentinvention. In this example, another structure as the wavelength lockingoptical system is adopted. That is, a collimated light beam of the laserdiode 100 is spatially divided into two portions of light whose boundaryis an optical axis 201, one of the two portions, as it is, being made toarrive at the photo detector 110 to produce the optical output monitorsignal. The other of the two portions passes through the etalon 108 toarrive at the photo detector 109 to produce the wavelength deviationdetection signal. Similarly, in this optical system, a resonator isformed between the facet of the laser diode and the etalon 108, and thesame problem as that described above may occur. To devise acountermeasure against the problem, the suppression of the resonancesimilar to that described above is performed with a combination of thepolarizer 113 and the wave plate 111, both of which are arranged in theoptical path. The photo detectors 109 and 110 can be monolithicallyfabricated, and that, with the width of an isolation region ranging froma few μm to a few tens of μm.

FIG. 10 illustrates another embodiment according to the presentinvention. This example is one that uses a slightly-tilted etalon.

In this example, a collimated light beam from the laser diode 100 ismade to enter the etalon 108 after being diverged or converged to acertain degree. The photo detectors arranged just after theslightly-tilted etalon 108 are split photo detectors as described withreference to FIG. 9. The spectral characteristics of two light beamseach passing through two parts of the slightly-tilted etalon 108 thatcorrespond to split photo detecting areas of the split photo detectorsare much the same curve as depicted in FIG. 4, but are different inphase (wavelength shift) relative to each other, and the subtraction ofboth characteristics produces the wavelength deviation detection signal.Also, in this wavelength locking optical system, resonance may occurbetween the etalon 108 and the facet of the laser diode, and hence, acountermeasure against the resonance needs to be devised. For thispurpose, this example performs polarization separation using acombination of the quarter-wave plate 111 and the polarizer 113, or onlywith the quarter-wave plate, to suppress the resonance. In this case,the optical output monitor signal of the laser diode can be a sum of thesignals of the photo detectors 110 and 109.

FIG. 11 is a plan view of an example of the laser diode light source 100that is used in accordance with the present invention. In this example,a DFB type laser diode 300 and an electro absorbing modulator 301 aremonolithically integrated. As shown in FIG. 1, the entire light sourceis mounted on a temperature controlling device, such as the Peltierdevice, and hence, the wavelength thereof can be tuned to a desiredwavelength by using the relation between the wavelength and thetemperature, as shown in FIG. 2. Note that, in tuning the wavelength,the temperature of the electro absorbing modulator 301 also changes;accordingly, it is not likely that the characteristic of the modulator301 is assured. Therefore, the present invention adopts a contrivancewhere a thin film heater 302 is formed adjacent to the optical modulator301 to control the operating temperature of the optical modulator 301always at an optimal value.

FIG. 12 illustrates another embodiment of the present invention. Thisexample uses a laser diode device in which a plurality of laser partsare integrated.

That is, DFB lasers 600 are integrated in the form of an array, opticaloutputs of the respective lasers are multiplexed into a single waveguideby a multiplexer 602 after passing through waveguides 601, and theoptical outputs are restored to a desired optical output by asemiconductor amplifier 603, regaining losses of the optical outputshitherto being experienced. Then, this light arrives at an exit facet604 through the electro absorbing modulator 301 equipped with the thinfilm heater 302. Subsequently, the light becomes a divergent light beam,is focused by the collimator lens 101, passes through the opticalisolator 102, is transmitted through the beam splitter 106, and isfocused onto the optical fiber 104 by the converging lens 103 so as totravel therein for information transmission. Reflected light from thebeam splitter 106 arrives at the photo detector 109 via the etalon orthe dielectric multi-layer filter 108. Here, a point that differentiatesthe present invention from the conventional configuration is that theoptical isolator 102 is arranged between the beam splitter 106 and thecollimator lens 105. With such a configuration, the returned light fromthe etalon 108 by reflection is blocked by the optical isolator 102 andno resonator is formed externally; therefore, no ringing occurs at thephoto detector 109. On the other hand, just behind the DFB lasers,waveguide photo detectors 605 are provided, which are used formonitoring the respective outputs of the DFB lasers.

FIG. 14 is a view showing an example of a communication module packageaccording to the present invention. FIG. 13 is a plan view showing anexample of the sub-assembly that is to be used therefor. That is, thissub-assembly is such that the wavelength locking optical systemaccording to the present invention is loaded in a communication modulepackage 700 of a butterfly type with 14 pins. The etalon 108, the waveplate 111, the beam splitter 106, the polarizer 113, and the photodetectors 109, 110 are mounted on a substrate 702, which is arranged onan optical path of the collimated light optical system of the laserdiode that is operated at a different site and is lasing at the desiredwavelength, and the incident angle of light to the etalon is adjustedand fixed to complete the sub-assembly. Following this, a sub-assembly701 is loaded in the optical path of the collimated optical system for adesired wavelength that has been produced in the package 700. Thismethod introduces an advantage of packaging that was made possiblethrough the use of the collimated plane wave in accordance with thepresent invention.

Making the wavelength locking part in the sub-assembly is hard toachieve by a method for wavelength-locking described in theabove-mentioned JP-A-79723/1998. The reason for this is that, in orderto adjust the angle of the divergent light beam which is to enter intothe etalon, the gap between the laser diode and the collimator lensneeds to be fine tuned, and the angle of the tilted etalon needs to befine tuned while monitoring the outputs from paired photo detectors.Consequently, it is necessary to carry out adjustment in a narrow,work-in-process module. The embodiment of the wavelength locking opticalsystem described above uses the etalon as a wavelength deviationdetection filter by way of example, but instead of the etalon, adielectric multi-layer filter can be used. In this case, polarizationcontrolling elements are employed to suppress the resonance between thedielectric filter and the facet of the laser diode.

As described above in detail, adoption of the configuration composed ofbasic elements according to the present invention as a solution makes itpossible to close a control loop for wavelength-locking that is stableoptically and electrically and hence to achieve the full performance ofthe etalon. That is, the present invention achieves resolution of atechnical problem that hitherto was impossible to solve, in terms ofresolution of wavelength selection, utilization of light, an increase inthe density of packaging inside the laser source module, mechanicalstability, correction of shift in locked wavelength due to thetemperature change, etc. Further, with the utilization of thewavelength-controlling scheme according to the present invention, itbecomes possible not only to lock the lasing wavelength of a laserdiode, but also to shift and then lock the wavelength thereof to any ofthe ITU-T grids. Therefore, according to the present invention, a lasersource indispensable for a wavelength division multiplexing opticalcommunication apparatus and a router device of wavelength channels canbe provided.

Hereafter, main forms by which the present invention is carried out inpractice will be enumerated.

A first form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects an amount of the lasing wavelengthshift of the laser source in accordance with the difference of photocurrents flowing in the respective photo detectors, and controls thewavelength of the laser source so that the difference of the photocurrents is kept at a constant value. A laser diode module and anapplication device in which the laser diode module is employed arecharacterized in that the laser diode device has means for reducingmutual superposition between an initial light beam and a reflected lightbeam generated by a reflecting substance existing in the light sourceand/or the optical path of the focused light beam in terms of physicaloverlapping, or reducing the mutual superposition in terms of thepolarization state.

A second form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents is kept at a constant value. A laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form are characterized in that awave plate is arranged in the optical path of the focused light beam.

A third form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents is kept at a constant value. A laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form are characterized in that awave plate and a polarizer are arranged in the optical path of thefocused light beam.

A fourth form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents is kept at a constant value. A laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form are characterized in thatthe wave plate arranged in the optical path of the focused light beamdoubles as light beam dividing means.

A fifth form of the present invention includes, in an optical module,wherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects -a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents equals a constant value. A laser diode module and anapplication device in which the laser diode module is employed accordingto the above-mentioned first form are characterized in that a wave plateand a polarizing beam splitter are arranged in the optical path of thefocused light beam.

A sixth form of the present invention includes, in an optical modulewherein a divergent light beam, which is-guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,and there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents is kept at a constant value. A laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form are characterized in that anoptical isolator consisting of a polarizer and a Faraday rotator isemployed.

A seventh form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents equals a constant value. The is also provided meansthat reduces mutual superposition between the initial light beam and areflected light beam generated by a reflecting substance existing in thelight source and/or the optical path of the focused light beam in termsof physical overlapping or mutual superposition described above in termsof polarization state. A laser diode module and an application device inwhich the laser diode module is employed are characterized in that thewavelength filter is a Fabry-Perot etalon.

An eighth form of the present invention includes, in an optical modulewherein a divergent light beam, which is guided directly or indirectlyfrom the rear or the front of a laser source or a light source in whichthe tunable laser source and a modulator are integrated, is focused witha lens, and a wavelength filter is arranged in an optical path of thefocused light beam, there is provided optical path dividing means fordividing transmitted light or reflected light of the wavelength filter,there is provided means that guides the divided light beams to aplurality of photo detectors, detects a quantity of the lasingwavelength shift of the laser source in accordance with the differenceof photo currents flowing in the respective photo detectors, andcontrols the wavelength of the laser source so that the difference ofthe photo currents equals a constant value. There is also provided meansthat reduces mutual superposition between the initial light beam and areflected light beam generated by a reflecting substance existing in thelight source and/or the optical path of the focused light beam in termsof physical overlapping or mutual superposition described above in termsof polarization state. A laser diode module and an application device inwhich the laser diode module is employed, characterized in that thewavelength filter is a dielectric multi-layer filter.

A ninth form of the present invention includes a laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form, characterized in that aplurality of laser sources each having the laser wavelength controllingmeans are arranged side by side and there is provided means thatcombines tunable wavelength ranges covered by respective laser sources.

A tenth form of the present invention includes a laser diode module andan application device in which the laser diode module is employedaccording to the above-mentioned first form, characterized in that partof or all of the laser wavelength controlling means is built into apackage of the laser source module.

An eleventh form of the present invention includes a laser diode moduleand an application device in which the laser diode module is employed,comprising at least: a laser diode device; a first photo detector partfor receiving, directly or indirectly, first divided light that is afirst portion of light when at least one of light emissions of the laserdiode device is divided into two portions of light, each traveling adifferent optical path; a second photo detector for receiving saidsecond divided light that is a second portion of light thus divided viaat least a wavelength selective member; and means for controlling thelasing wavelength of the laser diode device on the basis of outputs ofthe above-mentioned first and second photo detector parts, andcharacterized in that superposition between a beam of the light emissionfrom the above-mentioned laser diode device and a beam of the reflectedlight from a reflecting substance in the optical path between theabove-mentioned laser diode device and the above-mentioned wavelengthselective member is smaller than superposition of the beams at aparallel reflection plane.

According to the present invention, stabilization of the wavelength ofthe laser diode, namely, stabilization of a wavelength locking opticalsystem in the laser diode module comprising the wavelength lockingoptical system can be achieved.

Further, according to the present invention, external feedback noise inthe laser diode can be eliminated or alleviated.

What is claimed is:
 1. A laser diode module comprising: a laser diodedevice; a first photo detector part for receiving, directly orindirectly, first divided light that is a first portion of light when atleast one of light emissions of said laser diode device is divided intotwo portions of light each traveling a different optical path; a secondphoto detector part for receiving second divided light that is a secondportion of light thus divided via at least a wavelength selectivemember; and means for controlling the lasing wavelength of said laserdiode device on the basis of outputs of said first and second photodetector parts, wherein at least one of a quarter-wave plate and apolarizer is provided in an optical path between said laser diode deviceand said wavelength selective member.
 2. A laser diode module,comprising: a laser diode device; a first photo detector part forreceiving, directly or indirectly, first divided light that is a firstportion of light when at least one of light emissions of said laserdiode device is divided into two portions of light each traveling adifferent optical path; a second photo detector part for receivingsecond divided light that is a second portion of light thus divided viaat least a wavelength selective member; and means for controlling thelasing wavelength of said laser diode device on the basis of outputs ofsaid first and second photo detector parts, wherein a structure thatgenerates reflected light having a degree of polarization different fromthat of incident light falling on said wavelength selective member isprovided in an optical path between said laser diode device and saidwavelength selective member.
 3. A laser diode module, according to claim2, wherein said structure provided in the optical path enablesdirections of polarization of the incident light falling on saidwavelength selective member and of the reflected light to be orthogonalto each other.
 4. A laser diode module, comprising: a laser diodedevice; a first photo detector part for receiving, directly orindirectly, first divided light that is a first portion of light when atleast one of light emissions of said laser diode device is divided intotwo portions of light each traveling a different optical path; a secondphoto detector part for receiving second divided light that is a secondportion of light thus divided via at least a wavelength selectivemember; and means for controlling the lasing wavelength of said laserdiode device on the basis of outputs of said first and second photodetector parts, wherein a quarter-wave plate is provided in an opticalpath between said laser diode device and said wavelength selectivemember.
 5. A laser diode module according to claim 4, further comprisinga polarizer provided in the optical path between said laser diode deviceand said wavelength selective member.
 6. A laser diode module accordingto claim 4, wherein said quarter-wave plate forms a portion of a memberwhich enables division of at least one of light emissions of said laserdiode device into two portions of light each traveling a differentoptical path.
 7. A laser diode module according to claim 4, furthercomprising a polarizing beam splitter provided in the optical pathbetween said laser diode device and said wavelength selective member. 8.A laser diode module according to claim 4, wherein there are provided apolarizer and an optical isolator in the optical path between said laserdiode and said wavelength selective member.
 9. A laser diode moduleaccording to claim 4, wherein said wavelength selective member is atleast either of a Fabry-Perot etalon or a dielectric multi-layer filter.10. A laser diode module according to claim 4, wherein said laser diodedevice has a plurality of light emission regions.